The hard copying for image duplication generally relates to the screening and plate-making technology for printers and the advanced printing and plate-making device. The screening technology used for duplicating an image with hard copying is also called the digital image halftone technology. The digital image halftone technology comprises amplitude-modulation (AM) screening and frequency-modulation (FM) screening. The amplitude-modulation screening technique is also called ordered dithering of gathered dots, characterizing in that, the colored points in a produced halftone image are gathered in pairs geometrically to form clusters of colored regions, which are called dots. Since the technology controls the size of the dots to represent the gray level of the original image, the dots are called amplitude-modulation dots.
In the prior art there are some multi-bit depth imaging apparatus. The output gradations of the apparatus increase up to the power of 2 depending on the imaging depth. The general 2-bit or 4-bit output apparatus can represent 4 or 16 gradations, respectively. The final object of the multi-bit depth imaging apparatus is to obtain the output effect with high resolution under low resolution so as to overcome the problem of imaging quality under low resolution. Furthermore, in combination with the Pulse Width Modification (PWM) gradation offset technology embedded in the multi-bit depth imaging apparatus, the halftone dots are output stably and the optimized quality of the halftone dots output by the apparatus is obtained.
In view of the characteristics of the multi-bit depth apparatus, new demands and challenges for different ways of halftone screening emerge. The amplitude-modulation dot with one-bit depth is usually used in the conventional halftone screening process. Therefore, it is important for improving the high-quality amplitude-modulation effect of the multi-bit depth apparatus that how to transfer an one-bit amplitude-modulation dot data to the multi-bit amplitude-modulation dots associated with the characteristics of the apparatus, how to make full use of existing characteristics of the apparatus, and to obtain the output effect with high resolution under low resolution.
At present, image zoom technology is usually applied to achieve the multi-bit depth amplitude-modulation. With the image zooming technology, first the original one-bit depth amplitude-modulation dot data may be expanded to one-bit dot with a high resolution. Then the one-bit dot with a high resolution may be transferred to lattice data with a lower resolution supported by the current apparatus through the image zooming technology. Meanwhile, dot gradation required by the bit depth of the apparatus may be defined so as to achieve the multi-bit depth dots effect. However, it is required to transfer the dots data for many times and thus this method is too complicate to achieve and has a big problem of timeliness.